A reference ellipsoid is a mathematically defined surface that approximates the shape of the Earth. It is used in geodesy and cartography to provide a consistent framework for mapping and navigation. The ellipsoid is characterized by its semi-major and semi-minor axes, which represent the longest and shortest diameters, respectively. These ellipsoids help in converting geographic coordinates (latitude and longitude) into a three-dimensional Cartesian coordinate system. Common examples of reference ellipsoids include the WGS 84 and GRS 80, which are widely used in global positioning systems and various mapping applications.